The invention relates generally to electrical power distribution components. More particularly, the invention relates to automatic reclosers.
The basic functions of an electrical power system are to continuously maintain an adequate and reliable supply of electric power to customers. However, performing these functions is not always possible because various types of failures occur randomly and beyond the control of system engineers. Power system planners, designers, and operators are generally concerned with the reliability of their systems and the determination of realistic availability targets for their systems. Recently, this concern has been accentuated by the introduction of competition in the marketplace among utility companies. Now, utility companies are jockeying among themselves to provide distinguishable services in an effort to sustain existing customers and to attract new customers.
As a result, power system operators are constantly developing improved components for power distribution systems that will enhance the system""s reliability. For example, many power utilities around the world use fuses on their lateral lines. In addition to fuses, utilities may employ an automatic power restoration device or recloser. A recloser is used in an electrical distribution system to protect high voltage power lines by minimizing power distribution interruptions caused by transients or faults.
In operation, a recloser acts as an intelligent circuit breaker. In this sense, a recloser opens or trips a circuit when a certain event occurs; usually this event is the appearance of current, voltage, and/or frequency in a line. In one sense, when the recloser detects an overcurrent, i.e., current exceeding a predetermined threshold, the recloser will trip. The recloser serves to isolate the faulted portions of the line by opening that portion. The recloser then closes the open portion of the circuit after a predetermined period of time, usually when the appearance of current, voltage, and/or frequency disappears.
A recloser control device, which can be an electronic control circuit, operates a recloser. A recloser is usually mounted to a pole or tower that suspends power lines above the ground. However, use with below ground systems is also known.
More particularly, reclosers are electromechanical devices, similar to circuit breakers. Reclosers are distributed at one or more locations along a power line, typically upline from a fuse. When a fault condition is detected by the recloser controller, the recloser will begin to timeout. In other words, the recloser controller will initiate a trip to open the recloser if the fault condition has not cleared itself during a fixed time interval, where the time interval is a function of current. Then, after a time delay, as the name suggests, the recloser will close, and if the fault condition has been cleared, power service is restored. If, however, the fault condition has not been cleared, the recloser controller will again trip open the recloser after a second fixed time interval. If, after a predetermined number of reclose operations, the fault condition has not been cleared, the recloser controller will permanently lockout the recloser (i.e., permanently open the circuit). The circuit then remains open until the system is repaired and/or the fault condition is eliminated.
Thus, a primary function of a recloser is to save fuses. In general, this is done by sensing the peak value of the current conducted and interrupting its flow by opening or tripping a recloser before a fuse blows. After an interval or time delay, the recloser closes, thereby restoring power to the system where it remains closed until the next fault is sensed.
The rate at which a fuse will blow and interrupt current is a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the power generated by the fault and each fuse has a time current characteristic which describes the time interval required to interrupt the fault current. The time interval is generally approximately inversely proportional to the value of the root mean square of the fault current. It is desirable to coordinate the recloser with the fuses to be saved to insure that the recloser in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally done by approximating the root mean square value of the fault current by sensing its peak value.
It must also be recognized that not all faults which occur on a power distribution line are temporary, such as those caused by a branch momentarily falling against the line. Some faults are of a more permanent nature such as those caused by a line falling to the ground. As a consequence, reclosers are built so that they will only trip a limited number of times within a short duration before locking open. Were this not done, a recloser would cycle until failure and many of the fuses to be protected would blow anyway.
At some magnitude of fault current it is desirable to have the recloser open immediately to protect the line rather than following an inverse time current characteristic. At intermediate levels it may be desirable from the power distribution standpoint to allow the fault current to flow for a limited period to allow the fault to burn itself open or blow the fuse. Many reclosers have alternate inverse time current characteristics which achieve this goal. Typically, a recloser will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
Conventional reclosers for three phase systems open all three phases at the same time upon detection of a fault on any one phase. Additionally, a recloser can partially trip. That is, the recloser trips only one phase or two phases of three phases, on a three phase electrical power line. For example, if a fault is detected on only one phase of a network distribution, the recloser will open on that phase only and leave the remaining phases operational. For a two phase fault, the recloser opens only those two phases where the fault occurs and leaves the remaining phases operational. This feature allows an electric utility to prevent unnecessary three phase power outages in their distribution networks when the interruption is only single phase or two phase.
However, when a recloser trips only a single phase or two phases, a current imbalance in the distribution network can result. This current imbalance in turn can cause higher neutral/ground currents. The increase in neutral/ground current could be overcurrent and can appear as a ground fault to the system. Overcurrent in the neutral/ground line results in a three phase trip. Therefore, it is desirable to prevent a three phase trip after a partial trip of a recloser because the design of a partial trip is to prevent a three phase trip.
Therefore, a need exists for apparatuses and methods for automatically adjusting the current threshold of a neutral/ground line after a partial trip, thereby preventing a three phase trip.
The present invention is directed to apparatuses and methods for preventing a three phase trip of a power line in a power distribution network delivering electrical power through a plurality of current carrying conductors and a neutral/ground line. The three phase trip is prevented after a change in the current in the neutral/ground line caused by a partial trip of a recloser.
In an exemplary method, a partial trip in the power distribution network is detected, and then a threshold current is automatically adjusted from a first level to a second level for the neutral/ground line. The threshold current represents the amount of current the neutral/ground line will draw before activating a three phase trip.
The above-listed features of the present invention will be more fully set forth hereinafter.